1. Field of the Invention
The present invention relates to an image forming apparatus having mechanism by which sheets transported one by one are switch-backed, which is typified by a printer, a copier, or the like.
2. Description of the Related Art
An image forming apparatus typified by a printer, a copier, or the like, which can perform automatic duplex printing by switch-backing and reversing sheets transported one by one, has been put to practical use. In such an image forming apparatus, a sheet handling device such as a finisher having an after treatment device that connects to a back stage of the image forming apparatus such as a stacker having a paper tray, a stapler or the like requires a function of exhausting a sheet of which one side has been already printed in a state where its printed side is faced downward (hereinafter referred to as face-down exhaust) in order to readily make up print pages. Further, in case that sheets having binding holes are used, in order to exhaust the sheets in such a manner that the binding-hole directions at the one-sided printing time and at the two-sided printing time are fitted to each other, there is required a function of selectively switching that the sheet of which one side has been already printed is exhausted face-down, and the sheet of which two sides have been already printed is exhausted in a state where the last printed side is faced upward to be exhausted (hereinafter referred to as a face-up exhaust). Accordingly, in a recent image forming apparatus, a sheet reversing device, which has the function of reversing the surface of the already printed sheet and the rear surface and the function of selectively switching the face-up exhaust and the face-down exhaust, is necessary and indispensable in a last paper exhausting section.
FIG. 4 shows one example of the constitution of a paper feeding section in the conventional image forming apparatus that satisfies the above functions. In FIG. 4, a fixing device 9 provided in an image forming apparatus 1 is composed of a heat roller 9a and a pressure roller 9b that are pressure welded to each other, and a toner image transferred onto a sheet is fixed by this device 9. Reference numeral 10 is a gate and has a function of switching the directions of a sheet to be transported. A sheet transporting passage 12 connecting the fixing device 9 and a paper outlet 11 of the image forming apparatus to each other is referred to as a sheet feeding passage. Further, reference numeral 13a is referred to as a pull-in passage and diverges from the sheet feeding passage 12 on the downstream side in the sheet transporting direction in relation to the fixing device 9, and the sheet fed out from the fixing device 9 by the switching control of the gate 10 is selectively pulled in it. Further, reference numeral 13b is referred to as a reversed sheet exhausting passage and diverges from a midway divergent point P of the pull-in passage 13a, and its end joins the sheet feeding passage 12 on the upstream side of the sheet outlet 11 in the image forming apparatus 1. In FIG. 4, reference numeral 15 is a feed roller pair provided in the front stage of the gate 10, reference numeral 16 is a discharge roller pair in the back stage of the gate 10, and reference numeral 17 is a drive roller of a feed roller pair that is provided on the pull-in passage 13a and the reversed sheet exhausting passage 13b. Reference numerals 17 and 17a constitute an induction roller pair by which the sheet inducted to the pull-in passage 13a is put in, and reference numerals 17 and 17b constitute a feed roller pair for feeding out the sheet reversed and transported from the reversed sheet exhausting passage 13b. Reference numeral 18 is a reversal roller pair that is provided on the pull-in passage 13a and can switch its rotation to forward rotation/reverse rotation.
Under the above constitution, in case that the already printed sheet is exhausted face-up, the sheet fed out from the fixing device 9 is intactly exhausted along the sheet feeding passage 12 from the paper outlet 11, and fed out to a sheet handling device 2. Further, in case that the already printed sheet is exhausted face-down, the sheet fed out from the fixing device 9 is pulled in the pull-in passage 13a by guide of the gate 10, the reversal roller pair 18 is rotated in the opposite direction to the direction at the pulling-in time at a timing when a sheet rear end reaches the divergent point P in the pull-in passage 13a, and the sheet is transported from the reversed sheet exhausting passage 13b to the discharge roller pair 16 side and exhausted from the paper outlet 11 to be fed out to the sheet handling device 2 side.
In the above conventional art, in a device that transports sheets at a high speed, such a state occurs frequently that while a preceding sheet is reversed and transported on the reversed sheet exhausting passage 13b, a succeeding sheet is inducted to the pull-in passage 13a. In case that the sheet that is being reversed and transported and the succeeding sheet cross on the pull-in passage 13a, the following problem occurs.
In case that the rear end of the sheet during reversal passes through the reversal roller pair 18 and the succeeding sheet achieves the reversal roller pairs 18 before the rotation of the reversal roller pair 18 is switched to the forward rotation, the succeeding sheet stops in a state where its leading end portion cannot enter in the reversal roller pair 18, so that sheet jam occurs.
In order to solve the above problem, in the conventional image forming apparatus, the following methods are proposed and executed. As a first method, the induction roller pair 17, 17a is constituted by the drive roller different from the drive roller of the feed roller pair 17, 17b, the transporting velocity at the downstream of the sheet reversing device is heightened, and the reverse rotation velocity of the reversal roller pair 18 is made speedier than the forward rotation velocity thereof thereby to quickly exhaust the reversed sheet. As a second method, nip release mechanism that releases a nip of the reversal roller pair is provided, the nip is released before the succeeding sheet reaches the reversal roller, and two sheets that move in the opposite direction to each other can be existed between the reversal roller pair 18. As a third method, the distance between the preceding sheet and the succeeding sheet is made large.
However, according to the conventional first method, the transporting velocity at the downstream of the sheet reversing device becomes so high in the device transporting the sheets at the high speed that the driving force necessary for sheet transportation becomes large. Further, since consumption of the abrasive system of the sheet transporting device is severe, reliability of the device lowers. Further, since the velocity of the sheet exhausted from the sheet outlet 11 is different between a case that the sheet is reversed and a case that the sheet is not reversed, the sheet handling device 2 must correspond to the velocity of two kinds, so that the constitution of the device becomes complicated. Further, according to the conventional second method, the nip release mechanism so constituted that the drive roller constituting the reversal roller pair 18 is fixed and a pinch roller opposed to the drive roller is constituted separably in relation to the drive roller is simple and cheap in constitution. However, since the drive roller is exposed on the transporting passage, in case that the sheet that moves in the direction opposite to the rotational direction of the aforesaid drive roller passes, the sheet leading end is bent or stained due to action of the relative motion between the drive roller and the sheet, so that there is fear that bad influences on the succeeding sheet transportation and on the print quality occur. As a method for solving this fear, there are a system in which the drive roller is provided separably in relation to the pinch roller, and a system in which the roller on a side corresponding to the pull-in passage 13a is used as the pinch roller and the roller on a side corresponding to the reversed sheet exhausting passage 13b is used as the drive roller. According to the former system, the constitution becomes complicated as described above, so that a high cost of the apparatus is caused. Further, according to the latter system, since the pull-in passage 13a is located generally on the internal side of the apparatus, the nip release mechanism is arranged on the internal side of the apparatus, so that disadvantage that maintenance working becomes complicated occurs. Further, in case that the nip release operation is executed in relation to each sheet, there is fear that reliability lowers due to noise caused by the nip release operation and abrasion of the nip release driving system. Further, according to the conventional third method, since the distance of the sheets to be transported becomes large, through put lowers. Further, in case that delay of the sheet transportation occurs, this third method may not cope with the above problem, so that there is fear that reliability of the apparatus lowers.
An object of the invention, in order to solve the above defects of the conventional art, is to provide an image forming apparatus having a sheet reversing and exhausting device. This sheet reversing and exhausting device can perform switch back reversal without increasing the transporting velocity after sheet reversal and without releasing a nip in case of a sheet size that is highest in use of frequency, and actuates a nip release operation only in case that the nip release is necessary at a minimum. Further, the sheet reversing and exhausting device further can control the transporting velocity so that after the reversed sheet completely passed through a reversal roller pair also in case that transportation delay of the sheet occurs, a succeeding sheet reaches the reversal roller pair; and also in case that a sheet having a length requiring the nip release is switch-backed and reversed, this sheet reversing and exhausting device can prevent bending and a stain of the sheet that accompany the nip release.
To achieve the above object, according to the invention, there is provided an image forming apparatus having a sheet reversing device that comprising: a pull-in passage which is provided on the way of a first sheet transporting passage connecting a fixing device and an after treatment device to each other, and into which a sheet transported from the first sheet transporting passage is pulled; a reversal roller pair that is provided on the pull-in passage and provided so that it can rotate forward and reversely in order to reverse the advance direction of the sheet; a nip release means for releasing a nip of the reversal roller pair; a first roller pair for transporting the sheets to a reversal passage that diverges from the pull-in passage at a divergent point located upstream of the reversal roller pair and to the pull-in passage; and a second roller pair for transporting the sheet transported from the reversal passage by reverse rotation of the reversal roller pair. And, the sheet reversing device feeds out the sheet from the reversal passage in such a manner that a rear end of the sheet transported on the first transporting passage becomes a leading end in the transporting direction. In the invention, there are provided the sheet reversing device so constituted that the following expression is satisfied, and a control means which, in case of a sheet having a length in the transporting direction of at least 8.5 inch or less, causes the sheet to reverse without the nip release of reversal roller pair:
Y greater than L1 greater than S greater than {8.5 (inch)xe2x88x92X}/2+Vxc2x7tY greater than L2 
in which Y (inch) is the smallest length in the transporting direction of a sheet used in the image forming apparatus, X (inch) is a distance between a rear end of a preceding sheet and a leading end of a succeeding sheet during sheet transportation, L1 (inch) is a length of the transporting passage between the first roller pair and the reversal roller pair, L2 (inch) is a length of the transporting passage between the reversal roller pair and the second roller pair, S (inch) is a length of the transporting passage between the divergent point and the reversal roller pair, V(inch/sec.) is a sheet transporting velocity in the first sheet transporting passage, and t(sec.) is a time necessary for a deceleration/stop/acceleration step when the rotation of the reversal roller is switched from the forward rotation to the reverse rotation or from the reverse rotation to the forward rotation.
Further, a first sensor is provided on the first transporting passage of downstream of the fixing device of the image forming apparatus and it senses that the sheet has passed through the first roller pair. And, there is provided a control means, which compares a sheet rear end sensing moment by the first sensor with a regular moment, and which, in case that the rear end sensing moment is later than the regular moment, makes the transporting velocity up to the divergent point of the pull-in passage and the reversal passage and the reversing and transporting velocity between the divergent point and the second roller pair higher than the regular transporting velocity.
Further, a roller of the reversal roller pair on a side corresponding to the pull-in passage is constituted as a drive roller and a roller on a side corresponding to the reversal passage is constituted as a pinch roller, and the pinch roller is constituted separably from the drive roller. Further, there is provided a control means which, after the leading end of the sheet was sensed by the first sensor, rotates forward the drive roller of the reversal roller pair before a time when the sheet leading end reaches the reversal roller pair, which is calculated from the position of the reversal roller pair on the pull-in passage, passes.
According to a first aspect of the invention, since a length of the transporting passage between the first roller pair and the reversal roller pair and a length of the transporting passage between the reversal roller pair and the second roller pair are set smaller than the smallest length in the transporting direction of the sheet used in the image forming apparatus, a time till the rotation of the reversal roller pair is switched to the forward rotation after the rear end of the sheet having a length in the transporting passage of 8.5 inch after reversal has passed through the reversal roller pair can be set shorter than a time when the leading end of the succeeding sheet inducted into the pull-in passage reaches the reversal roller pair. Therefore, the sheet having a length in the transporting passage of at least 8.5 inch or less, in a state where it is always nipped by the rollers, can be switch backed and reversed without the nip release of the reversal roll.
According to a second aspect of the invention, since the transportation delay of the sheet before the sheet is inducted to the sheet reversing device can be sensed by the first sensing means, it is possible to correct the transportation delay of the sheet by correcting the transporting velocity of the reversal roller pair in the section where the sheet is transported by only the reversal roller pair.
According to a third aspect of the invention, before the timer starting a clocking operation from the moment when the sheet leading end is sensed by the first measuring means points to a time when the sheet leading end reaches the reversal roller pair, which is calculated from the transporting velocity of the sheet and the distance between the first measuring means and the reversal roller pair, the rotation of the reversal roller pair is switched to the forward rotation.